Reduced particle contamination manufacturing and packaging for reticles

ABSTRACT

A method of transporting a reticle is disclosed. The reticle is placed in a reticle carrier that has an ionizer. Moreover, the reticle may be attached with a pellicle. The pellicle consists of a pellicle frame and a pellicle film stretched over the pellicle frame. The pellicle frame has included within an absorbent material.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of, and claims priority from,U.S. patent application Ser. No. 09/737,125, filed Dec. 13, 2000, whichis a divisional of, and claims priority from, U.S. patent applicationSer. No. 09/477,795, filed Dec. 30, 1999, U.S. Pat. No. 6,279,249.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to photolithography ofsemiconductor devices, and more particularly, to a method of making andpackaging reticles resulting in reduced particle contamination.

[0004] 2. Background Information

[0005] Photolithography is a process that is commonly used in themanufacture of integrated circuits. The process involves the depositionof a photoresist layer onto an underlying substrate layer. Thephotoresist is then selectively exposed to light, which chemicallyalters the photoresist. The photoresist is then developed and thoseportions of the photoresist that are exposed to light are eitherhardened or softened, depending upon whether or not the photoresist isnegative or positive photoresist, respectively.

[0006] The pattern that is projected onto the photoresist layer iscontained on a mask that is placed within the photolithography exposuretool. A mask, also referred to as a reticle, is placed between theilluminating light and the photoresist. The reticle is typically formedfrom patterned chromium placed on glass or quartz. The pattern istransferred onto the photoresist by projecting an image of the reticleonto the photoresist using an exposing radiation.

[0007] In many applications, the reticle is covered by a pellicle. Apellicle is a thin film of optical grade polymer that is stretched on aframe and secured to the reticle. The pellicle's purpose is to preventairborne dirt from collecting on the mask and acting as an opaque spot.During the exposure, the dirt is held out of the focal plane and doesnot print on the wafer.

[0008] With the need for smaller critical dimensions, photolithographytechnology has evolved into using extreme ultraviolet (EUV) exposureradiation that has a smaller wavelength. One wavelength that is becomingpopular is 157 nm. Unfortunately, current pellicles are formed from amaterial that are either not sufficiently transparent to radiation at157 nm or does not have sufficient durability under these processconditions. Therefore, in many applications, reticles are manufacturedwithout pellicles. This causes increased risk to contamination of thereticle.

[0009] Moreover, after the reticle has been manufactured, the reticlemust be transported from the manufacturer to the semiconductorfabrication facility (known as a “fab”). This transport processincreases the risk of contamination. Currently, reticles are stored andshipped in reticle containers without any devices specifically designedto remove particles from the environment in which they are enclosed.

SUMMARY OF THE INVENTION

[0010] A method of transporting a reticle is disclosed. The reticle isplaced in a reticle carrier that has an ionizer.

BRIEF DESCRIPTION OF DRAWINGS

[0011] The present invention will be described in conjunction with thefollowing Figures, wherein:

[0012]FIG. 1 is a schematic diagram illustrating a pellicle carrierformed in accordance with the present invention;

[0013]FIG. 2 shows a flow diagram illustrating the steps of handling areticle in accordance with the present invention;

[0014]FIG. 3 shows a flow diagram illustrating the steps of mounting apellicle onto a reticle in accordance with the present invention;

[0015]FIGS. 4a-c show schematic diagrams of a pellicle formed inaccordance with the present invention;

[0016]FIG. 5 shows a flow diagram illustrating the steps of modifying areticle and pellicle combination in accordance with the presentinvention;

[0017]FIG. 6 shows a flow diagram illustrating the steps of modifying areticle and pellicle combination in accordance with an alternativeembodiment of the present invention;

[0018]FIG. 7 shows a flow diagram illustrating the steps of modifying areticle and pellicle combination in accordance with another alternativeembodiment of the present invention; and

[0019]FIG. 8 is a schematic diagram of a pellicle frame used with themethod of FIG. 7.

DETAILED DESCRIPTION OF THE INVENTION

[0020] The present invention offers a way to purify the environment in areticle carrier to prevent contamination. In accordance with the presentinvention, the reticle carrier includes an ionizer that produces ions.Because particles and other contaminents often hold a net electricalcharge allowing electrostatic forces to attract and hold particles tothe surface of the reticle, the ions act to neutralize the contaminantswithin the reticle carrier. This prevents the contaminants from adheringto reticle surfaces, and thus mitigating the need for a pellicle.

[0021] Specifically, as a means to prevent particles from collecting onpellicle-less mask surfaces, the mask is cleaned and inspected forpattern fidelity, hard defects, and soft defects, including particlesand contamination. The mask is placed in a super clean mini-environmentpod, such as a standard mechanical interface (SMIF) pod. The SMIF pod isa self contained, portable mini-environment that surrounds semiconductorproduction wafers and minimizes exposure to contaminants that couldadversley affect chip performance and yield. The above steps are allconventionally performed in the prior art.

[0022] However, in accordance with the present invention, the SMIF podis modified to include an ionizer that generates ions for neutralizingcharged contaminants. Specifically, turning to FIG. 1, a SMIF pod 101 isshown. The SMIF pod 101 includes an ionizer 103, a pair of oppositelycharged particle collectors 105, and a reticle 107. The ionizer 103generates ions that will neutralize contaminants within the SMIF pod101. Moreover, the particle collectors 105 are oppositely charged. Thisresults in the contaminants being attracted to one or the other of theparticle collectors 105. Note that in FIG. 1, the reticle 107 ispreferably placed with the chromium side 109 down to prevent particlesfrom landing on the particle sensitive side of the reticle.

[0023] The entire process of handling the manufactured reticle is shownin FIG. 2. First, at box 201, the reticle is placed faced down into theSMIF pod 101. Next, at box 203, the SMIF pod 101 is subsequently placedin a transport box while still inside the clean room. The transport boxis then shipped to outside wafer fabs. Upon arrival at the wafer fab, atbox 205, the transport box is wiped down per standard cleaning protocolsand moved to inside the clean room. At box 207, the mini-environmentSMIF pod 101 is removed from the transport box and either moved to thelithography tools if needed immediately for wafer production or intostorage if not.

[0024] When the reticle is required, at box 209, the SMIF pod 101 isinterfaced directly with the lithography tool so that the reticle isnever exposed directly to factory ambient air. Automatic transfer andloads of the reticle is done into the photolithography stepper. Finally,at box 211, in situ laser cleaning is performed prior to wafer exposureto remove adsorbed substances. The light source may be either the oneused to expose the wafers or a separate source.

[0025] In accordance with the other aspects of the present invention, amethod is provided for removing transmission inhibiting chemical speciesduring the manufacturing process of the reticle. Typically, opticalcontaminants exist adsorbed on the reticle and pellicle surfaces, in gasphase in the space between the reticle and the pellicle, or both. In aconventional reticle manufacturing process, a pellicle is mounted on thepatterned side of the reticle to protect it from contamination. Thismounting process is invariably performed in a clean room environment toprevent particles from being introduced into the process.

[0026] Because the mounting process is perfomed in atmospheric air, thegas space between the pellicle and reticle (hereinafter referred to asthe “interpellicle space”) will contain air. Common atmosphericsubstances include carbon and oxygen containing species such as O₂, CO₂,and H₂O, which are known to inhibit the transmission of radiation havingwavelengths commonly used in the photolithography process, such as 157nm. These species may exist: (1) in the gas phase in the interpelliclespace, (2) adsorbed on the reticle surface, including either thesubstrate or thin film surfaces, (3) adsorbed on and absorbed in thepellicle, (4) or adsorbed on the pellicle frame surfaces.

[0027] The transmission of 157 nm wavelength radiation can be reduced bythe presence of even monolayer quantities of such chemical species.Removal of these species or substitution of these species with othermaterials which do not absorb the exposing radiation wavelength istherefore required for photolithographic processes, especially those inthe extreme ultra-violet region and below.

[0028] Therefore, what is provided is a method for mounting a pellicleonto a reticle under an optically inert gas environment. The completedreticle can then be transferred to a photolithography tool. The methoddescribed herein is not dependant upon the gas environment used andtherefore can be applied with a variety of substances. Preferably, thegas environment should be optically inert to the exposing radiationwavelength.

[0029] In accordance with the present invention, referring to FIG. 3,after the reticle has been patterned, cleaned, and its quality verifiedto specification requirements, the reticle (referred to as a mask inFIG. 3) is transferred into a process chamber with a pellicle at step301. The chamber is evacuated and maintained at vacuum or filled with anoptically inert gas (such as nitrogen) at approximately atmosphericpressure (plus or minus 50 psi). By optically inert gas, it is meant agas that has minimal or negligible effect on the transmission ofradiation at the wavelength of the exposure radiation used in thephotolithography stepper. Next, at step 303, the mask and pelliclesurfaces are then exposed to either (1) UV radiation, (2) plasma, (3)ozone, (4) heat, or (5) a combination of the above. This exposure willremove adsorbed and absorbed species. In other words, the treatment willdrive desorption. If an optically inert gas is used rather than vacuum,it is flowed during this process to carry away desorbed substances. Theoptically inert gas, such as nitrogen, may also be heated to acceleratethe desorbtion process.

[0030] The time of exposure at step 303 will depend upon the finalexposure radiation transmission level required, the exposure optionused, the pellicle material chosen, the gas flow rate or vacuumpressure, the convective mass transfer coefficient (if gas is used), andthe species adsorbed and absorbed on the incoming photo mask. When thetargeted exposure time is reached, the treatment of step 303 (of the UVradiation, plasma, ozone, heat, etc.) is discontinued. The gas flow orvacuum maybe simultaneously discontinued or allowed to continue for aperiod of time. The exact operating process and its optimization woulddepend on the equipment geometry, materials, flow path and other designperimeters.

[0031] Next, at step 305, once the target exposure time is reached, thechamber is filled with an optically inert gas (if vacuum was used insteps 301 and 303) or the gas flow is stopped (if optically inert gaswas used). Next, at step 307, robotic systems mount the pellicle eitherin situ or after automated transfer to another station (all under anoptically inert gas environment).

[0032] The assembled reticle is moved to a particle inspection tooleither through a mini-environment pod (such as a SMIF pod) or through arobotic transfer system. In either case, the reticle is maintained in anoptically inert environment. As an option, the particle inspection stepcan be skipped, provided that the risk of particle contamination issufficiently low.

[0033] At step 309, the reticle is maintained in an optically inert gasatmosphere during subsequent storage and shipping by means ofmini-environment SMIF pods. As seen in step 311, loading ontophotolithographic tools can be achieved by use of robotics systems whichtransfer the reticle from the pod to the tool and serve as an interfacewhich maintains the optically inert gas environment surrounding thephoto mask.

[0034] As an additonal protection against transmission inhibitingcompounds, polar and hydrogen bonding absorbance can be embedded in thepellicle frame to act as a scavenger for any traces of unwanted gases.Turning to FIGS. 4a-c, a design for a pellicle along these lines isshown.

[0035] As seen in FIGS. 4a-c, the pellicle 401 includes a pellicle frame405 and a pellicle film 403. The pellicle film 403 is stretched over thepellicle frame 405. The frame is filled with an absorbent material. Theabsorbent material may be one of the well known polar and hydrogenbonding absorbent materials. The absorbent may be present in one or allsides of the pellicle frame. The absorbent acts as a scavenger for anytraces of unwanted gases. Note that the shape and packing density shownin FIG. 4c is only for illustrative purposes. Note that the interior orexterior frame wall may be constructed or coated with a solid adsorbentmaterial as well.

[0036] The above description provides an improved method formanufacturing and attaching the pellicle to the reticle. For someapplications, the reticle has already been attached to a pellicle. Evenfor these situations, the present invention provides a method forpurifying these “pre-made” pellicle/reticle combinations. By purifying,it is meant the removal of the interpellicle air and substitution withan optically inert gas.

[0037] There will be described three different methods for substitutingthe air in the interpellicle space with an optically inert gas. Themethods described here are not dependant on the gas used and thereforecan be applied with a variety of substances at the users discretion.

[0038] Referring to FIG. 5, the first method is referred to as the“vacuum assisted removal” technique. After the reticle has been attachedto the pellicle and all of the manufacturing and inspection steps havebeen completed, at step 501, the reticle (attached with a pellicle andreferred to as “pelliclized”) is transferred to a process chamber thathas been evacuated. Evacuation of the interpellicle gas during vacuumpump down relies on a pressure release valve (PRV) as a fluid conduit.The PRV is normally integrated into current pellicle frames to equalizeambient pressure with the pressure in the interpellicle space. The size,number, and placement of PRV's may be changed from existingconfigurations to accommodate this new evacuation function. The PRV maybe as simple as a single small orifice in the frame.

[0039] After a suitable vacuum has been achieved, at step 503, thepelliclized reticle is exposed to either ultra violet radiation, plasma,ozone, heat, or a combination of the above to remove the adsorbed andabsorbed species. The vacuum is augmented with UV or these other methodsto drive desorbtion and removal of contaminants. The time of exposurewill depend on the final transmission level required and other factorsnoted above. This step 503 is similar to step 303 described above.

[0040] Next, at step 505, once the target exposure time is reached, thechamber is filled with an optically inert gas, such as nitrogen, untilatmospheric pressure is reached. The rate of the gas fill will bedependent on the rate that the PRV devices allow gas to enter theinterpellicle space. Next, at step 507, the pelliclized reticle is movedinto a mini-environment pod (such as a SMIF pod) through a robotictransfer system. In either case, the reticle is maintained in anoptically inert environment.

[0041] At step 509, the reticle is maintained in an optically inert gasatmosphere during subsequent storage and shipping by means ofmini-environment SMIF pods. As seen in step 509, loading ontophotolithographic tools can be achieved by use of robotics systems whichtransfer the reticle from the SMIF pod to the tool and serve as aninterface which maintains the optically inert gas environmentsurrounding the reticle.

[0042] In the second embodiment referred to as “passive removal” andshown in FIG. 6, after the reticle has been pelliclized and completedall of its manufacturing and inspection steps at step 601, it istransferred to a process chamber at step 603 where it is exposed to acontinuous flow of optically inert gas and either UV radiation, plasma,ozone, heat, or a combination of the above. The flowing gas creates achemical potential gradiant of the transmission inhibiting compoundsacross the reticle/gas phase interface to drive mass transfer. Thechemical potential gradiant is augmented with ultra-violet or othermeans to drive desorbtion and removal of contaminents. To accelerate thedesorbtion kinetics, the gas also may be heated to a higher temperatureprior to chamber introduction. Like the above methods, the time ofexposure will depend on various factors discussed above.

[0043] Next, at step 605, once the target exposure time is reached, thechamber is filled with an optically inert gas, such as nitrogen, untilatmospheric pressure is reached. Next, at step 607, the pelliclizedreticle is moved into a mini-environment pod (such as a SMIF pod)through a robotic transfer system. In either case, the reticle ismaintained in an optically inert environment.

[0044] At step 609, the reticle is maintained in an optically inert gasatmosphere during subsequent storage and shipping by means ofmini-environment SMIF pods. As seen in step 609, loading ontophotolithographic tools can be achieved by use of robotics systems whichtransfer the reticle from the SMIF pod to the tool and serve as aninterface which maintains the optically inert gas environmentsurrounding the reticle.

[0045] Finally, in the third method referred to as “convective removal”and shown in FIG. 7, after the reticle has been pelliclized at step 701and completed all of its manufacturing and inspection steps, at step703, it is transferred to a chamber where optically inert gas is routedthrough the interpellicle space through a frame which has been modifiedto receive and distribute gas while the reticle exterior is exposed to acontinuous flow of gas. Additionally, either UV radiation, plasma,ozone, heat or a combination of the above is used to remove the adsorbedand absorbed species. The inert gas flows are augmented with UV or othermeans to drive desorbtion and remove all contaminents. Once again, thetime of exposure will tend to depend upon the final transmission levelrequired.

[0046] Turning to FIG. 8, a pellicle frame 801 formed in accordance withthe present invention is shown. The pellicle frame 801 includes amanifold that connects a gas source 803 to the interpellicle space. Thepellicle frame 801 is equipped with a valve device 805 for flowmodulation and shutoff as well as a filter 807 for particulate control.The valve 805 and filter 807 replaces the traditional PRV devices.

[0047] Next, at step 705, once the target exposure time is reached, thechamber is filled with an optically inert gas, such as nitrogen, untilatmospheric pressure is reached. Next, at step 707, the pelliclizedreticle is moved into a mini-environment pod (such as a SMIF pod)through a robotic transfer system. In either case, the reticle ismaintained in an optically inert environment.

[0048] At step 709, the reticle is maintained in an optically inert gasatmosphere during subsequent storage and shipping by means ofmini-environment SMIF pods. As seen in step 709, loading ontophotolithographic tools can be achieved by use of robotics systems whichtransfer the reticle from the SMIF pod to the tool and serve as aninterface which maintains the optically inert gas environmentsurrounding the reticle.

[0049] While the preferred embodiment of the invention has beenillustrated and described, it will be appreciated that various changescan be made therein without departing from the spirit and scope of theinvention.

What is claimed is:
 1. A method, comprising: providing a pellicle and areticle into a process chamber, wherein the pellicle is not mounted tothe reticle; exposing the pellicle and the reticle to a treatment fordesorption of optical contaminants that may be adsorbed on or absorbedin the pellicle and the reticle; introducing an optically inert gas intosaid process chamber; and mounting the pellicle onto the reticle.
 2. Themethod of claim 1 wherein the treatment comprises exposure to at leastone of ultraviolet radiation, ozone, heat, and plasma.
 3. The method ofclaim 1, further comprising evacuating the process chamber.
 4. Themethod of claim 1 wherein during the treatment, the process chamber isin a vacuum state.
 5. The method of claim 1, further comprising flowingthe optically inert gas through the process chamber during thetreatment.
 6. The method of claim 5, further comprising heating theoptically inert gas.
 7. The method of claim 1 wherein the opticallyinert gas is nitrogen.
 8. The method of claim 1 wherein the pellicle ismounted onto the reticle in an optically inert gas environment.
 9. Themethod of claim 1, further comprising placing the pellicle mounted ontothe reticle in a standard mechanical interface (SMIF) pod.
 10. Themethod of claim 1 wherein the pellicle includes a pellicle filmstretched over a pellicle frame.
 11. The method of claim 10 wherein thepellicle frame includes a manifold formed within the pellicle frame.